This invention relates generally to the treatment of used foundry sands, and more particularly, to the reclamation of foundry sands including both clay-bonded and organic resin-bonded sands and mixtures thereof.
Foundry sand is commonly used to make core molds into which ferrous and non-ferrous metals are cast. The core molds consist of sand bonded with special additives including inorganic binders such as clay, and organic resin binders, such as phenol, melamine, or urea formaldehyde.
Previously, after the casting has set within the mold, the mold was broken away and discarded. Various factors such as the deplation of natural sand deposits, the cost of disposing of used sand in accordance with recent environmental regulations have now made it economical and advantageous to reclaim the used sand for repeated use.
The type of binder used has, in the past, generally dictated the type of reclamation process utilized. Thus, for the inorganic, clay-bonded sands, so called "wet" and "dry" scrubbing techniques have been employed. U.S. Pat. No. 2,261,947 to Barnebl et al issued Nov. 11, 1941, entitled "Foundry Practice", utilizes the wet scrubbing method in which clay-bonded sands are cleaned in a blasting room by means of a high pressure stream of water and sand projected from guns. In the "dry" scrubbing technique, the "dry" sand is projected against an abrasive surface to crack off the clay binder. In both cases, the cleaning action is that of mechanical attrition.
For those sands utilizing the organic resin binders, thermal reclamation techniques have commonly been employed. For example, U.S. Pat. No. 2,,478,461 to Connolly issued Aug. 9, 1949, entitled "Apparatus And Method For Treating Foundry Sand", discusses the reclaiming of foundry sand by heating or roasting treatment which causes the organic binders to be burned away. Thermal methods have commonly employed multiple-hearth type furnaces with mechanical "rabble arms" which worked over the used sand and moved the sand through the hearth at elevated temperatures.
None of the previously described techniques have proved entirely satisfactory for use in reclaiming both the clay-bonded and organic resin-bonded sands, as where a mixture of the sands is present. "Dry" scrubber reclaimers presently operating on clay-bonded sands have been found to have low rates of efficiency. This condition shows up as a gradual build-up of "binders" on the sand grains to the point where up to 50 percent of the reclaimed sand must be discarded over a period of time. "Wet" scrubbers, although more effective on clay-bonded sands, are not effective on organic resin bonded sands. Conversely, previous thermal techniques have proved effective for organic resin-bonded sands but not for clay-bonded sands. In addition, the rabble-roaster type multiple hearth furnace used previously is expensive to set up and maintain due to the necessity for operating the mechanical moving parts in high temperature environments and due to high initial cost of the high alloy, heat resistant metal components.